1. Field of the Invention
The present invention relates to a thermal transfer recording apparatus and a facsimile apparatus for recording image on recording medium by transferring ink contained in an ink sheet to the aforesaid recording medium.
2. Related Background Art
Generally, a thermal transfer printer uses an ink sheet with heat meltable (or heat sublimable) ink coated on the base film thereof, and selectively heats such ink sheet by the thermal head in response to image signals in order to transfer the molten (or sublimated) ink to a recording sheet for image recording. Usually, an ink sheet of the kind is such that the contained ink is completely transferred to the recording sheet for one image recording (the so-called one-time sheet). Therefore, it is necessary to convey the ink sheet for an amount equivalent to the length of recorded one character or one line of image after the image recording has been completed, so that the unused portion of the ink sheet should reliably be brought forward to the position for the next recording. Thus the consumption of the ink sheet becomes great and the running cost of the thermal transfer printer tends to be higher than that of a usual thermal printer using thermal sheets for recording.
With a view to solving a problem such as this, a thermal transfer printer has been proposed, in which both recording sheet and ink sheet are conveyed in the same direction at different speeds, as disclosed in Japanese Laid-Open Patent Applications Nos. 57-83471 and 58-201686 or Japanese Patent Publication No. 62-58917. As described in the aforesaid publications, an ink sheet (multiprint sheet) capable of recording images for plural numbers (n) is known. When a length L of recording is continuously performed using this ink sheet, it is possible to carry on the recording by making the length of ink sheet to be conveyed after each image recording has been completed or during the image being recorded shorter than the length L by (L/n:n&gt;l). Hence the ink sheet can be used more efficiently than the conventional sheet by n times, and it is therefore expected that the running cost of the thermal transfer printer is lowered. Hereinafter this recording method is referred to as multiprint.
In the conventional multiprint, however, said n value is constant irrespective of printing modes. In the case of a thermal transfer printer generally in use, the faster the recording speed is, the greater is the ratio of the period to energize the thermal head. Consequently, the temperature of the thermal head is raised, so that ink contained in the ink sheet is easily molten or sublimated. As a result, if this ink sheet is employed for a facsimile apparatus or the like for example, the recording density becomes thin for a superfine mode, etc. necessitating a slower recording speed, whereas the recording density is thick for a standard mode which is a higher speed recording. On the contrary, if the recording is performed just fine with this ink sheet in the superfine mode, the density becomes too high in the standard mode, and there is a possibility that the recorded image is smeared.
Also, in a facsimile apparatus, etc., when the transfer speed is fast, requiring a shorter cycle of scanning or recording, the heat is accumulated on the thermal head to cause the thermal head to generate a higher temperature. Accordingly, the image transfer becomes easier because ink contained in the ink sheet is molten. On the other hand, if the transfer speed is slow, making the cycle of scanning or recording longer, the thermal head is cooled at each of the intervals between the recording periods, thus making it difficult to transfer ink contained ink sheet.
In the conventional apparatus, however, the length (n) to convey the ink sheet against the recording sheet is always fixed for a constant value as described earlier. Therefore, there is a possibility that the amount of ink transfer of the ink sheet varies due to such variations of recording cycle, etc., and that the densities of recorded images vary to lower the image quality.
Likewise, in a half tone mode, etc., for example, necessitating a slower recording speed, the cycle to energize the thermal head also becomes longer, so that the temperature of the thermal head is lowered. Then ink contained in the ink sheet tends to be difficult to be molten or sublimated. However, since the aforesaid n value is fixedly set for the above-mentioned thermal transfer printer, the relative speed between the recording sheet and ink sheet remains unchanged even in a state where it is difficult to transfer ink contained in the ink sheet. As a result, there is a possibility that the amount of ink transfer is reduced to cause the density of the recorded image to be lowered.
As set forth above in detail, there is a possibility that the image quality is lowered by the influence of heat accumulation when the recording mode (such as standard mode, fine mode, recording cycle, half tone mode, or the like) is shifted because the aforesaid n value is constant in the conventional multiprint.